The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

Was the distribution of galaxies throughout the universe random and uniform, as most astronomers assumed? Hubble himself had thought so. “On a large scale the distribution is approximately uniform,” he had written in his highly influential 1936 book, The Realm of the Nebulae. “Everywhere and in all directions, the observable region is much the same.” In a sense, he was simply reiterating the two assumptions of modern cosmology, homogeneity and isotropy, in layman’s terms. But the way he was framing the issue was also reminiscent of the premodern island-universe thinking—emphasis on “island.” In Hubble’s view, and therefore the view of a generation of astronomers, the galaxy clusters that astronomers had observed would be accidents of nature, or perhaps a sort of cosmic optical illusion arising from multiple galaxies falling along our line of sight. But Gamow was thinking on a different scale. Maybe the peculiar motions of galaxies—the motions that were separate from a straightforward outward expansion—weren’t random, as most astronomers assumed. Maybe the gravitational interactions among galaxies, even across previously unthinkable distances, were sometimes strong enough to counteract the expansion on a local level. Maybe no—or at least not every—galaxy is an island.